Acids (OCR A Level Chemistry)

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Acids, Bases & Dissociation

Strong acids dissociating 

  • Strong acids will fully dissociate in solution 
  • For example, when hydrogen chloride (HCl) is dissolved in water 100% of the product will be ions. 
    • HCl (aq) → H+ (aq) + Cl- (aq)
    • In this case hydrogen ions are released, H+ (aq) 
    • The same applies with strong bases

Strong bases dissociating

  • Strong bases will fully dissociate in solution
    • NaOH (aq) → Na+ (aq) + OH- (aq)
    • In this case hydroxide ions are released, OH- (aq) 

Weak acids dissociating

  • Weak acids only partially dissociate in solution, only a small percentage of the products will be ions
  • In an equilibrium reaction, the products are formed at the same rate as the reactants are used
  • This means that at equilibrium, both reactants and products are present in the solution
  • For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
    • CH3COOH (aq) CH3COO- (aq) + H+ (aq)
    • The same applies with weak bases

Weak bases dissociating

  • Weak bases only partially dissociate in solution, only a small percentage of the products will be ions
  • An equilibrium is established containing reactants and products
  • For example, ethylamine (CH3CH2NH2) is a weak base and will partially dissociate in solution and produce hydroxide ions 
    • CH3CH2NH(aq) + H2O (l) ⇌ CH3CH2NH3+ (aq) + OH- (aq) 
Examples of acids and bases 

2-3-1-examples-of-acids-and-bases

Neutralisation

  • A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt

Equilibria Neutralisation Reaction, downloadable AS & A Level Chemistry revision notes

  • The proton of the acid reacts with the hydroxide of the base to form water

Equilibria Neutralisation Reaction Ions, downloadable AS & A Level Chemistry revision notes

  • The spectator ions which are not involved in the formation of water, form the salt

Equilibria Neutralisation Reaction of HCl and NaOH, downloadable AS & A Level Chemistry revision notes

The diagram shows a neutralisation reaction of HCl and NaOH and the two individual reactions that take place to form the water and salt

  • The name of the salt produced can be predicted from the acid that has reacted

Acid Reacted & Salt Table

Equilibria Table 1_Neutralisation Reactions, downloadable AS & A Level Chemistry revision notes

Metals and acids

  • The typical reaction of a metal and an acid can be summarized as

acid + metal    salt + hydrogen

  • For example:

2HCl (aq) + Zn (s)   ZnCl2 (aq) +   H2 (g)

hydrochloric acid + zinc   zinc chloride + hydrogen

  • Clearly, the extent of reaction depends on the reactivity of the metal and the strength of the acid
  • Very reactive metals would react dangerously with acids and these reactions are not usually carried out
  • Metals low in reactivity do not react at all, for instance copper does not react with dilute acids
  • Stronger acids will react more vigorously with metals than weak acids. What signs of reaction would be expected to be different between the two?
    • Faster reaction, seen as
      • more effervescence
      • the metal dissolves faster
      • more exothermic

Metals and oxides

  • The reaction of an acid with a metal oxide forms two products:

acid + metal oxide   salt + water

  • For example:

2HCl (aq) + CaO (s)      CaCl2 (aq) + H2O (l)

  hydrochloric acid + calcium oxide   calcium chloride + water

Metals and hydroxides

  • The reaction with a metal hydroxide and an acid follows the same pattern as an oxide:

acid + metal hydroxide   salt + water

  • For example

 H2SO4 (aq) + Mg(OH)2 (s)  → MgSO4 (aq)  + 2H2O (l)

 sulfuric acid  + magnesium hydroxide →  magnesium sulfate + water

Metals and carbonates

  • The reaction between a metal carbonate and an acid produces three products:

acid + metal carbonate salt + water + carbon dioxide

  • For example:

    2HNO3 (aq) + CuCO3 (s)→ Cu(NO3)2 (aq) + H2O (l) + CO2 (g)

nitric acid + copper carbonate copper nitrate + water + carbon dioxide

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.